One important type of automotive blind-spot warning systems employs short pulses of electromagnetic or ultrasonic energy to interrogate the detection zone. A decision regarding the presence or absence of an obstacle at a predetermined range is then made by suitably processing energy backscattered by various objects in the field of view of the system.
FIG. 1 is a block diagram of a typical obstacle-detection system utilizing short pulses of electromagnetic energy. The system comprises a pulse generator PGR that produces repetitively pulses with duration TP so selected as to provide required range resolution ΔR. The pulse repetition period TREP may be constant or may vary in some specified manner. The system also has an oscillator OSC that generates a sinusoidal signal with required carrier frequency, a pulse modulator PMD that modulates the carrier signal in an on-off fashion, a power amplifier PAM that amplifies the pulsed carrier signal to a required level, a transmit element TEL that radiates pulses of electromagnetic energy towards an obstacle OBS, a suitable receive sensor REL that receives electromagnetic pulses reflected back by the obstacle OBS, an input amplifier IAM that amplifies the signal provided by the receive sensor REL, a signal conditioning unit SCU that employs suitable signal processing to shape the received pulses, and a pulse-coincidence processor PCP that processes jointly the reference pulses supplied by the generator PGR and reconstructed pulses supplied by the signal conditioning unit SCU to provide a decision DEC regarding the presence or absence of an obstacle at a predetermined range.
Usually, the input amplifier IAM is blanked during pulse transmission intervals, in order to suppress an undesired leakage signal originating in the transmitter. The required blanking function is accomplished by applying pulses provided by the generator PGR to the blanking input BI of the amplifier IAM.
FIG. 2 is a block diagram of a multichannel pulse-coincidence processor PCP utilized by the obstacle-detection system of FIG. 1. The processor has a decision block DBK and a plurality of channels, each comprising a suitable delay unit DELN, a coincidence gate CG and a coincidence counter CCR. The plurality of delay values, DEL1, DEL2, . . . ,DELJ, corresponds to a plurality of range values of interest, referred to as range gates. In each channel, reference pulses provided by the generator PGR are suitably delayed and applied to one input of coincidence gate CG, whose other input is driven by pulses reconstructed by signal conditioning unit SCU. When a pulse coincidence occurs, the counter CCR adds a one to the already accumulated number of coincidences. At the end of a prescribed observation period, each coincidence counter CCR supplies the number G of accumulated pulse coincidences to the decision block DBK. The decision block DBK selects the greatest of the supplied numbers, G1, G2, . . . , GJ, and compares this maximum value with a suitable decision threshold DT. If the decision threshold has been exceeded, then the decision block DBK declares, at the output DEC, an obstacle present in the range gate that exhibits the greatest number of observed coincidences.
FIG. 3 depicts a periodic pulse sequence comprising rectangular pulses of duration TP and repetition period TREP. The range resolution depends on the pulse duration TP and the unambiguous range of the system depends on the period TREP.
It is known that target detectability can be improved significantly when a single pulse is replaced by a suitably constructed pulse packet. Consequently, a basic periodic pulse sequence, such as the one depicted in FIG. 3, can be replaced by a sequence of successive pulse packets (also referred to herein as pulse trains).
In this arrangement, each pulse packet comprises a specified number N of identical pulses which are staggered nonuniformly, with each interpulse spacing being an integer multiple of a suitably chosen unit time interval. The pattern of interpulse spacings is so designed as to ensure that only a small number ha of pulse coincidences (preferably at most one pulse coincidence) will occur between a primary pulse packet and its replica shifted in time by more than one pulse duration. This condition is usually referred to as the autocorrelation constraint.
Consider a pulse packet of span (length) L comprising N identical rectangular pulses of unit duration. Such a pulse packet can be conveniently represented by a binary sequence {x}=x1x2 . . . xL of symbols 0 and 1, in which symbol 1 corresponds to pulse occurrence. In this case, the autocorrelation constraint can be expressed as
                    R        xx            ⁡              (        d        )              =                            ∑                      i            =            1                                L            -            d                          ⁢                              x            i                    ⁢                      x                          i              +              d                                          ≤              h        a            <      N        ,          ⁢      0    <    d    ≤          L      -      1      where Rxx(d) is the autocorrelation sequence and d is the integer shift. When d=0, the autocorrelation value Rxx(0) simply equals the number N of pulses contained within the pulse packet.
In the class of all pulse packets with a specified number of pulses N and ha=1, a maximally compact pulse packet has the minimal span Lmin. Consequently, the maximally compact pulse packet exhibits the largest duty factor, N/L, and the largest average power. For a fixed N and ha=1, all pulse packets with spans greater than Lmin are referred to as sparse pulse packets.
FIG. 4 depicts a pulse packet of span L=36 comprising N=8 pulses which are placed at locations 1, 8, 11, 17, 19, 31, 32 and 36. The pulse packet can be represented by the following binary sequence {x}                {x}=100000010010000010100000000000110001The autocorrelation sequence Rxx(d) of {x} is shown in FIG. 5a. The peak value of Rxx(d) occurs at zero shift, i.e. Rxx(0)=8; for other shifts d, the function Rxx(d) assumes a value of either zero or one (ha=1). While the autocorrelation sequence Rxx(d) fully characterises the binary sequence {x}, the corresponding pulse packet is usually characterised by the autocorrelation function Rxx(τ), where the parameter τ denotes continuous time delay (shift). The autocorrelation function Rxx(τ) of the pulse packet represented by {x} is shown in FIG. 5b, where Δ denotes the unit time interval. Both the autocorrelation sequence Rxx(d) and the autocorrelation function Rx(τ) are even functions of their respective arguments.        
The autocorrelation constraint ensures that when there is no noise or interference, and a multichannel pulse-coincidence processor is used for detecting a pulse packet, the output of each channel is at most ha except when the channel delay matches that of a pulse packet being received. In this case, the channel output reaches the peak value of N.
In practical systems, in order to suppress undesired leakage from the transmitter, the receiver is usually blanked during pulse transmission intervals. The autocorrelation constraint Rxx(d)≦1 implies that when the pulse packet being received overlaps the pulse packet being transmitted, at most one received pulse in a target return will be lost.
In a multiuser environment, the users may transmit their signals. simultaneously and asynchronously so that not only must each receiver recognize and detect its own transmitted signal, but it must be able to do so in the presence of the other transmitted signals. Assume that a pulse packet to be detected by a receiver of interest is represented by a binary sequence{x}=x1 x2 . . . xLand that one of the interfering pulse packets is represented by another binary sequence{y}=y1 y2 . . . yL
In order to optimize the detection performance of the receiver in multiuser environment, the following cross-correlation constraints must be satisfied for all integer shifts d
                              R          xy                ⁡                  (          d          )                    =                                    ∑                          i              =              1                                      L              -              d                                ⁢                                    x              i                        ⁢                          y                              i                +                d                                                    ≤                  h          c                <        N              ,                  ⁢          0      <      d      ≤              L        -        1              and                              R          yx                ⁡                  (          d          )                    =                                    ∑                          i              =              1                                      L              -              d                                ⁢                                    y              i                        ⁢                          x                              i                +                d                                                    ≤                  h          c                <        N              ,                  ⁢          0      <      d      ≤              L        -        1            When more than one transmitter is in operation, the autocorrelation and cross-correlation constraints combined together ensure that, when there is no noise and a multichannel pulse-coincidence processor is used for detection, the output of each channel is still substantially less than N except when the channel delay matches that of a received pulse packet of interest.
Various techniques have been developed to construct sets of binary sequences with good autocorrelation and cross-correlation properties (see for example P. Fan and M. Darnell, Sequence Design for Communications Applications. Wiley, 1996). However, these are generally only of limited use in automotive obstacle-detection systems designated to operate in multiuser environment, as they would produce multiple different long sequences exhibiting a very low duty factor, hence the resulting detection performance will be significantly degraded.
In automotive applications, many similar obstacle-detection systems should be capable of operating in the same region, also sharing the same frequency band. To avoid mutual interference, each system should use a distinct signal, preferably uncorrelated with the signals employed by all other systems. Because it is not possible to predict which of the many similar systems will be operating in a particular environment, it is not practical to assign a distinct binary sequence to each of them. Furthermore, it is also very difficult to construct large sets of binary sequences with good autocorrelation and cross-correlation properties, and also exhibiting acceptable duty factors.